This will continue lines of ///doxygen comments when you reach the end of a line while typing a comment. It also works if you use o to open a new line while on a comment starting with ///. It only works if there is a space between the last / and the first letter of the comment, that is no big deal, since it lends itself to readability. So for example:

/// This will work.
///This won't.

Here is the magic line. Make sure you put it somewhere that will get sourced whenever you open a file you want to use with doxygen. I have it in ~/.vim/after/ftplugin/c/c.vim, so it gets sourced for all C and C++ files.